A known vehicle is provided with a drive unit that opens and closes an opening and closing member, for example, a back door (i.e., serving as an opening and closing member), a swing door, a slide door, and a window. For example, a first end portion of the drive unit that opens and closes the back door (tailgate) includes a coupling connected to a vehicle body, and a second end portion of the drive unit includes a coupling connected to the opening and closing member. The drive unit includes a threaded spindle, a spindle nut and a spindle tube. The threaded spindle rotates by motive power of a motor and human power (power) of an operator. The spindle nut is threaded with the threaded spindle. The spindle tube includes a first end portion which is fixed to the spindle nut and a second end portion which is fixed to the coupling connected to the opening and closing member.
In a case where the threaded spindle rotates by motive power, rotary motion of the threaded spindle is converted into linear motion of the spindle nut by the threaded spindle and the spindle nut. Accordingly, the spindle tube fixed to the spindle nut moves linearly to open and close the opening and closing member. In addition, the drive unit is constructed to open and close the opening and closing member manually by an operator.
The drive unit includes a compressive coil spring for holding the opening and closing member in an opened state. The compressive coil spring holds the opening and closing member in the opened state by generating reaction force which balances with the weight of the opening and closing member per se. As disclosed in JP4885910B (hereinafter referred to as Patent reference 1), a drive unit is provided with a resistance generating device which generates resistance against rotary motion of a threaded spindle in order to hold an opening and closing member in an opened state even in a case where an external force, for example wind or snow, is applied to the opening and closing member.
As disclosed in Patent reference 1, the resistance generating device for a drive unit includes the threaded spindle and a spindle nut. The resistance generating device disclosed in Patent reference 1 is provided with a fixing member, a ring magnet and a rotary member. The fixing member is made from non-magnetizable material, for example, plastic or aluminum. The ring magnet is disposed inside the fixing member. The rotary member is placed outside the fixing member. In a case where a motor is in a stopped state, magnetic force of the ring magnet attracts the rotary member toward the fixing member to have the rotary member contact the fixing member. Accordingly, resistance against the rotary motion of the rotary member is generated. On the other hand, in a case where the motor is in motion, the centrifugal force is applied to the rotary member and releases resistance against the rotary motion of the rotary member because the rotary member separates from the fixing member.
Another resistance generating device for a window, a door, a lid and a shelf of a building material, furniture, vehicle and machinery is disclosed in JP2005-265174A (hereinafter referred to as Patent reference 2). The resistance generating device disclosed in Patent reference 2 includes a fixing member and a rotary member. Each of the fixing member and the rotary member is provided with the north pole magnets and the south pole magnets which are alternately disposed with each other. The fixing member and the rotary member are disposed close to, and facing with each other. The rotary member rotates relative to the fixing member in a non-contact manner. The magnetic force generates resistance against rotary motion of the rotary member.
However, according to Patent reference 1, because resistance is generated by frictional force having an unstable characteristic, resistance force may be largely changed due to abrasion deterioration or wetting of a surface of the fixing member or the rotary member and due to extraneous materials (abrasion powder) attached to the surface of the fixing member or the rotary member. Further, friction noise may be generated in a case where the rotary member slides relative to the fixing member. Similarly, contact noise (for example, tapping sound) may be generated in a case where the rotary member comes in contact with the fixing member. In addition, a large number of members may be necessary to move the rotary member by an application of the centrifugal force, which complicates the construction of the resistance generating device.
According to Patent reference 2, because the rotary member rotates relative to the fixing member in a non-contact manner to generate resistance, the abrasion of the surface of the fixing member or the rotary member does not occur. However, the resistance generating device disclosed in Patent reference 2 costs high because both the rotary member and the fixing member are provided with magnets.
A need thus exists for a resistance generating device for a drive unit which is not susceptible to the drawback mentioned above.